Shock-absorbing intervertebral implant

ABSTRACT

The invention relates to an intervertebral implant comprising a spacer designed to be applied between two spinous processes of two vertebrae. The spacer comprises: two elements ( 10, 12 ) made of a first material, and each presenting a first end ( 10   a,    12   a ) and a second end ( 10   b,    12   b ), said first end ( 10   a,    12   a ) being securable to a spinous process; and a connection piece ( 14 ) made of a second material of greater elastic deformability than said first material, said connection piece interconnecting said second ends ( 10   b,    12   b ) of said two elements ( 10, 12 ) so that the stresses that are exerted on said two elements ( 10, 12 ) are damped, and said connection piece enabling said intervertebral implant to limit and brake the relative movements of said vertebrae.

[0001] The present invention relates to an intervertebral implantdesigned to be applied, in particular, between two spinous processes oftwo vertebrae.

[0002] The indications for which this type of implant is inserted andfixed between the spinous processes generally originate fromdeterioration of the intervertebral disk. In particular, when theposterior portion of an intervertebral disk has deteriorated, extensionof the spine causes the two vertebrae that are separated by the disk tomove towards each other abnormally. This generally causes the nerveroots to become trapped and causes the person who is affected by thisproblem to experience pain.

[0003] Intervertebral implants comprising a spacer that is insertedbetween the spinous processes and that includes fixing means are wellknown. These spacers, generally made of titanium alloy, present a notchat each of their ends, with the spinous processes being received in thenotches. In addition, the spacer is held by ties, interconnecting thetwo opposite edges of each of the notches and tightened around part ofthe wall of each spinous processes.

[0004] Such implants limit the extent to which the vertebrae can movetowards each other since, when the spine is in extension, the spinousprocesses tend to come into abutment against the bottoms of the oppositenotches in which they are inserted. However, the material of which thespacer is made is hard compared with the material of an intervertebraldisk which, when it is intact, limits the extent to which the vertebraecan move towards each other, so much so that the jolts which can betransmitted to the spine, e.g. while walking, are not damped between twovertebrae interconnected by a spacer. Furthermore, since the spacer doesnot have the same mechanical properties as the remaining portion of theintervertebral disk, the overall mechanical properties of the spinepresent significant discontinuities compared with an intact spine,thereby increasing deterioration of the intervertebral disk.

[0005] An object of the present invention is to provide anintervertebral implant having two opposite notches against which thespinous processes come into abutment, which notches present relativemobility with relative movements between them being damped.

[0006] To achieve this object the invention provides an intervertebralimplant comprising a spacer designed to be applied between two spinousprocesses of two vertebrae, the implant being characterized in that saidspacer comprises:

[0007] two elements made of a first material, and each presenting afirst end and a second end, said first end being securable to a spinousprocess; and

[0008] a connection piece made of a second material of greater elasticdeformability than said first material, said connection pieceinterconnecting said second ends of said two elements so that thestresses that are exerted on said two elements are damped, and saidconnection piece enabling said intervertebral implant to limit and brakethe relative movements of said vertebrae.

[0009] Thus, the connection piece situated between the two elements,each secured to a spinous process, tends to become compressed when thespinous processes move towards each other, and absorb the stressesexerted on said two elements. As a result, the vertebrae move towardseach other with a certain amount of elasticity that is close to thenatural elasticity conferred by an intact intervertebral disk.Furthermore, the relative elastic mobility of the vertebrae iscompatible with the elastic deformation of the posterior ligaments whichhold the vertebrae together. A system is thus obtained in which, understress, the relative mobility of its component elements is substantiallyidentical to the relative mobility of the elements of the originalintact system, thereby protecting the elements from furtherdeterioration.

[0010] In advantageous manner, said second ends of the spacer present asecuring wall onto which said connection piece is suitable for beingbonded. Thus, no additional fixing member is necessary and the adhesiveproperties of the second material co-operate with the securing wall.

[0011] In a particular embodiment of the invention, said securing wallpresents recesses that are suitable for co-operating with projections ofsaid connection piece in such a manner as to increase bonding betweensaid connection piece and said wall. It will be understood that becauserecesses are formed in a wall, the surface area of said wall isincreased, thereby increasing the area of contact between the twomaterials when one of the materials can be molded onto the wall of theother. The increase in contact area increases the connection forcesbetween said connection piece and said two elements. Furthermore,recesses are formed in such a manner as to increase contact area andalso to increase the static friction forces between said two elementsand the material of the connection piece, said forces being in additionto the connection forces.

[0012] Advantageously, said second material of said connection piece isconstituted by a body obtained by polymerization. As a result, theconnection piece can easily be hot molded onto said elements if thematerial is polymerized beforehand, or it can be constituted in situ ifthe monomers constituting said second material polymerize at a speedthat is slow enough to provide enough time to make the assembly.

[0013] In a preferred embodiment of the invention, said first materialof said elements is a titanium alloy. It is thus easy to form recessesin said securing wall onto which said connection piece is suitable forbeing bonded.

[0014] In a first particular embodiment, each first end of said twoelements forms a notch between two wings that is suitable for receivinga spinous process, and said implant further comprises a tie ofadjustable length interconnecting said two wings, said tie surrounding aportion of said spinous process in such a manner as to secure said firstend to said spinous process.

[0015] This characteristic of the intervertebral implant thus resides inthe way said elements are fixed onto the spinous processes. A tie ispreinstalled on each of said elements of said spacer, and once saidspacer is inserted between two processes, said elements are secured tothe processes by tightening said ties.

[0016] In a second particular embodiment of the invention, each firstend of said two elements forms a notch between two wings that issuitable for receiving a spinous process, and said implant furthercomprises a pin that is suitable for passing laterally through saidwings and said process in such a manner as to secure said first end tosaid spinous process.

[0017] In this configuration, the spinous processes are piercedtransversely, and the elements are connected thereto by means of a pinor rivet which passes both through the two wings of the element andthrough the process situated between them. The pin or rivet is fixedonto one or both of the wings in such a manner as to prevent it frombeing removed accidentally.

[0018] In a third particular embodiment of the invention, each first endforms a notch between two wings that is suitable for receiving a spinousprocess, and said implant further comprises a clip-forming semicircularpart interconnecting said wings, said clip surrounding a portion of saidspinous process in such a manner as to secure said first end to saidspinous process.

[0019] As a result, the clips are easily fixed onto the wings of saidelements once the spacer has been inserted. As explained in greaterdetail below, in this particular embodiment, since the wings aredisengaged, the spacer is inserted without major surgery on theposterior ligaments.

[0020] Other features and advantages of the invention will appear onreading the following description of particular embodiments of theinvention given by way of non-limiting indication with reference to theaccompanying drawings, in which:

[0021]FIG. 1 is an axial section view of an intervertebral spacer of theinvention;

[0022]FIG. 2 is a diagrammatic view in elevation showing theintervertebral implant provided with its adjustable fixing ties;

[0023]FIG. 3 is a diagrammatic view in elevation showing theintervertebral implant provided with fixing pins; and

[0024]FIG. 4 is a diagrammatic view in elevation showing theintervertebral implant provided with fixing clips.

[0025] The spacer and the method of connecting the elements thatconstitute it are described initially with reference to FIG. 1.

[0026] The intervertebral implant includes two symmetrical elements 10and 12 each presenting a first end 10 a and 12 a and a second end 10 band 12 b. The two elements 10 and 12 are made of a bio-compatiblematerial of the titanium-alloy type, suitable for remaining permanentlyinside the body on the spine.

[0027] Each first end 10 a, 12 a presents a notch 10′a, 12′a in which aspinous process is capable of bearing in such a manner that each firstend 10 a, 12 a surrounds substantially half of the circumference of aprocess, said process passing through the first end 10 a, 12 a.

[0028] The elements 10 and 12 are interconnected by a connection piece14, interposed between them, in such a manner that said elements 10 and12 are held symmetrically relative to each other. More precisely, it isthe two ends 10 b and 12 b of the elements 10 and 12 that areinterconnected.

[0029] The connection piece 14 is constituted by a body, obtained bypolymerization, of the plastics-material type. The body is selected frommaterials having elastic deformability that is greater than that of thematerial of said elements 10 and 12, and especially having elasticproperties that are similar to those of the posterior ligaments whichhold the various elements of the spine together.

[0030] Organic silicon compounds form polymers having mechanicalproperties that are capable of being determined by choosing their basicingredients, in particular by their degree of substitution, the natureof the substituents, and their molecular weight, and having elasticbehavior that is preponderant compared with their plastic behavior.Thus, they constitute a family of materials suitable for interconnectingsaid two elements 10 and 12. Furthermore, these polymers are capable ofbeing highly adhesive on materials of inorganic composition. Thus, whenthe elements 10 and 12 are made of titanium alloy, the connection piece14 provides a good connection.

[0031] However, suitable polymer-type materials are not restricted tothe organic silicon compounds and any other material presenting similarproperties could be used.

[0032] The material of said connection piece 14 is suitable for bondingonto a securing wall of each of said two substantially plane ends 10 band 12 b. However, in order to increase adhesion, recesses 16 are formedin the securing walls of the ends 10 b and 12 b and these recesses aresuitable for co-operating with projections 18 of the connection piece 14which are inserted in the recesses 16.

[0033] This characteristic serves, firstly to increase the contact areabetween the two materials, thereby increasing the connection forcebetween them in a direction that is normal to said contact surface, andsecondly to create static friction forces which are in addition to thebonding force.

[0034] Such a connection is made either by injecting the hot polymerbetween the two elements 10 and 12 held facing each other in a mold, orby molding a cold mixture of monomers between the two elements 10 and 12if their reaction speed is sufficiently slow. The projections 18 arethus formed in situ when the liquid or semi-liquid polymer inserted inthe recesses 18 solidifies after cooling or after chemical reaction. Itwill be understood that the connection piece 14 is constituted by thepolymer interposed between the elements 10 and 12, and that in order tomaintain it between the facing elements while it is in the liquid state,the walls of the mold must necessarily surround the space between thetwo elements 10 and 12 in line with them.

[0035] In a particular embodiment, not shown, the recesses 16 formed inthe securing wall open out into the outside wall of the elements 10 and12 so that the liquid polymer penetrates completely into the recesses 12without any air being trapped therein. As a result, the connectionbetween the material of the connection piece 14 and the elements isstrengthened.

[0036] In addition, the recesses, shown parallel to the longitudinalaxis of the spacer in FIG. 1, are capable of being formed obliquelyrelative to the longitudinal axis and/or of being non rectilinear. Theseconfigurations enable the static friction forces of the polymer on theelements 10 and 12 to be increased, thereby strengthening theirinterconnection.

[0037] In another embodiment of the invention, not shown, the elements10 and 12 are axially pierced in the securing walls of their second ends10 b and 12 b in order to form a single recess opening out into theirfirst ends 10 a and 12 a at the bottoms of their notches 10′a and 12′a.A portion of the recess-forming hole situated in the vicinity of thenotch presents a diameter that is greater than that of the hole whichopens out into the securing wall, in such a manner as to form ashoulder. The connection piece is thus molded between the elements 10and 12 in such a manner that the polymer-type material penetrates intothe two recesses and fills them completely as far as the bottom surfacesof their notches 10′a and 12′a. Furthermore, the diameter of the holesis greater than the diameter of the recesses shown in FIG. 1. Thus, oncethe material of the polymer type has set, it not only secures the twoelements 10 and 12 by means of its adhesive properties on the insidewalls of the holes, but it also secures them mechanically since theportions of material molded in said greater-diameter portions come intoabutment against said shoulders.

[0038] Particular embodiments of the invention are described below withreference to FIGS. 2, 3, and 4.

[0039]FIG. 2 shows a first particular embodiment of the invention inwhich the two opposite edges of the notches 10′a and 12′a form wings 20,21, 22, 23 interconnected in pairs by ties 30 and 32 each having aportion between said wings 20, 21, and 22, 23 of a length that isadjustable, and each being locked by the tension which can be applied onsaid ties 30 and 32.

[0040]FIG. 2 shows the two elements 10 and 12 connected by theconnection piece 14. The implant of the invention is inserted betweentwo spinous processes 34, 36 of two adjacent vertebrae in such a mannerthat the two notches 10′a and 12′a, facing in opposite directions,respectively engage part of the adjacent bottom portion of the upperspinous processes 34 and part of the adjacent top portion of the lowerspinous processes 36.

[0041] The ties 30 and 32 respectively pass round the top portion of theupper spinous process 34 and the bottom portion of the lower spinousprocess 36 in such a manner that the spinous processes 34 and 36 arecapable of being locked in the respective notches 10′a and 12′a. Lockingis performed by tightening the ties 30 and 32, which, by means of slotsmade in the wings 21 and 23, are trapped in said wings 21 and 23.

[0042] Thus, the elements 10 and 12 are secured to the respectiveprocesses 34 and 36, and relative movement of said processes 34, 36 ispossible within the deformation limits of the connection piece 14 madeof polymer-type material.

[0043] When the two spinous processes 34 and 36 move towards each other,in particular during extension of the spine, the two elements 10 and 12compress the connection piece 14 in elastic manner, i.e. the reactionforce which tends to hold the spinous processes 34 and 36 apart fromeach other is substantially proportional to the relative displacement ofthe two processes. Thus, the implant can take over the role of theintervertebral disk to be replaced (or the portion of the intervertebraldisk to be replaced) concerning the spacing that it serves to maintainbetween two vertebrae so as not to trap any roots. It also makes itpossible for resulting stresses which are applied to the spinousprocesses to be compatible with the stresses exerted on the processes bythe posterior ligaments.

[0044] Furthermore, when the two spinous processes 34 and 36 move awayfrom each other, when bending the spine, the connection piece issubjected to traction having a return force that is also substantiallyproportional to the elongation to which it has been subjected, at leastfor small amplitudes. The spine can thus bend through greater amplitudethan that which is possible when the two spinous processes are securedto each other in fixed manner.

[0045]FIG. 3 shows a second particular embodiment of the invention inwhich the wings of the notches, and the spinous processes which areinserted between them, are locked together by a pin which passes throughthe spinous processes.

[0046]FIG. 3 shows the intervertebral spacer between the two spinousprocesses 34 and 36, and the pairs of wings 20, 21 and 22, 23 whichengage parts of the processes 34 and 36. Furthermore, respective pins 40pass through the notches 10′a and 12′a as well as through the spinousprocess 34 and 36. The wings 20, 21 and 22, 23 are respectively piercedwith two facing orifices through which the pins 40 can be inserted andfixed via their ends.

[0047] Prior to inserting the implant between the spinous processes 34,36, said spinous processes are pierced laterally with respectiveorifices having a diameter that is greater than the diameter of the pins40. The intervertebral spacer is then inserted between the spinousprocesses 34, 36, and the rivet-forming pins 40 are then slid throughthe wings 20, 21 and 22, 23 and the corresponding spinous processes 34,36. The ends of the pins 40 are flattened longitudinally in such amanner as to give them a diameter that is greater than that of theorifices in the wings 20, 21 and 22, 23 through which they pass. In thisway, the pins 40 are locked in longitudinal translation relative to thespacer, and the two elements 10 and 12 are thus respectively secured tothe spinous processes 34 and 36.

[0048] In this second particular embodiment of the invention, it is notnecessary to remove the interspinous ligaments or to disinsert thesupraspinous ligaments that underlie and overlie the intervertebralspace in which the spacer is to be inserted. During insertion of thespacer, since the pairs of wings 20, 21 and 22, 23 are notinterconnected by means suitable for surrounding the spinous process, itis necessary to remove the interspinous ligaments and to disinsert thesupraspinous ligaments only from the intervertebral space in which thespacer is to be inserted.

[0049]FIG. 4 shows a third embodiment of the invention providing thesame advantages as those described above for the second embodiment.

[0050]FIG. 4 shows the intervertebral spacer between the two spinousprocesses 34 and 36, and the pairs of wings 20, 21 and 22, 23 whichengage parts of the processes 34 and 36. In contrast, the elements 10and 12 of the intervertebral spacer are secured to the spinous processes34 and 36 by means of respective clip-forming semicircular parts 42.

[0051] The semicircular part 42 includes two hook-forming ends that aredirected towards the inside of the part, and that are suitable for beinginserted into the orifices formed in the outside walls of the pairs ofwings 20, 21 and 22, 23. The part 42 is elastically deformable so thatit is force-fitted onto the elements 10 and 12.

[0052] Once the spacer has been inserted between the spinous processes34, 36, the clips 40 are fixed on the elements 10 and 12 in such asmanner that the inside walls of the semicircular parts 42 cover wallportions of the spinous processes that complement the wall portions ofthe spinous processes covered by the notches 10′a and 12′a. The spinousprocesses 34 and 36 are thus held in the notches 10′a and 12′a by theclips 42, thereby securing the elements 10 and 12 to said spinousprocesses.

[0053] The above-described embodiments of the invention are not limitedsolely to an intervertebral implant that is suitable for beinginterposed between two adjacent vertebrae. Thus, it is not beyond theambit of the invention to provide an implant that is constituted by twoimplants as described above, which are interconnected via the ends oftheir wings, along their longitudinal axis, in such a manner as to limitand brake the relative movements of three consecutive vertebrae.

1/ An intervertebral implant comprising a spacer designed to be appliedbetween two spinous processes (34, 36) of two vertebrae, the implantbeing characterized in that said spacer comprises: two elements (10, 12)made of a first material, and each presenting a first end securable to aspinous process (34, 36) and a second end (10 b, 12 b) presentingrecesses (16); and a connection piece (14) made of a second material ofgreater elastic deformability than said first material, said connectionpiece interconnecting said second ends (10 b, 12 b) of said two elements(10, 12), said connection piece presenting projections (18) suitable forbeing inserted in said recesses (16) so as to increase the bondingforces between the connection piece and said two elements, and saidconnection piece enabling said intervertebral implant to limit and brakethe relative movements of said vertebrae. 2/ An intervertebral implantaccording to claim 1, characterized in that said recesses (16) and saidprojections (18) with which they co-operate are oriented substantiallyparallel to the longitudinal axis of said spacer. 3/ An intervertebralimplant according to claim 1 or claim 2, characterized in that saidsecond material of said connection piece (14) is constituted by a bodyobtained by polymerization. 4/ An intervertebral implant according toany one of claims 1 to 3, characterized in that said first material ofsaid elements (10, 12) is a titanium alloy. 5/ An intervertebral implantaccording to any one of claims 1 to 4, characterized in that each firstend (10 a, 12 a) of said two elements (10, 12) forms a notch (10′a,12′a) between two wings (20, 21, 22, 23) that is suitable for receivinga spinous process (34, 36), and in that said implant further comprises atie of adjustable length (30, 32) interconnecting said two wings (20,21; 22, 23), said tie (30, 32) surrounding a portion of said spinousprocess (34, 36) in such a manner as to secure said first end (10 a, 12a) to said spinous process (34, 36). 6/ An intervertebral implantaccording to any one of claims 1 to 4, characterized in that each firstend (10 a, 12 a) of said two elements forms a notch (10′a, 12′a) betweentwo wings (20, 21, 22, 23) that is suitable for receiving a spinousprocess (34, 36), and in that said implant further comprises a pin (40)that is suitable for passing laterally through said wings (20, 21, 22,23) and said process (34, 36) in such a manner as to secure said firstend (10 a, 12 a) to said spinous process (34, 36). 7/ An intervertebralimplant according to any one of claims 1 to 4, characterized in thateach first end (10 a, 12 a) forms a notch (10′a, 12′a) between two wings(20, 21, 22, 23) that is suitable for receiving a spinous process (34,36), and in that said implant further comprises a clip-formingsemicircular part (42) interconnecting said wings (20, 21, 22, 23), saidclip (42) surrounding a portion of said spinous process (34, 36) in sucha manner as to secure said first end (10 a, 12 a) to said spinousprocess (34, 36).